Crystal oscillator and resonator



Aug. 1933. TILLYER 1,924,297

CRYSTAL OSCILLATOR AND RESONATOR Filed Aug. 4, 1930 VACUUM PUMP Patented Aug. 29, 1933 longer CRYSTAL OSCILLATOR AND RESONATOR Edgar D. Tillyer, Southbridge, Mass, assignor to Radio Corporation of America, New York, N. Y.,

a Corporation of Delaware Application August l, 1930. Serial No. 473,083

Claims.

This invention relates to crystal control radio transmitters and has particular reference to an improved process and apparatus for controlling the oscillations of a crystal or piezoelectric os- 5 cillator or resonator.

The principal object of this invention is to provide means and processes whereby the frequency of a crystal or piezo-electric oscillator or resonator may be regulated to a required amount.

Another object is to provide means whereby the change in frequency of oscillations of the crystal or piezo-electric oscillator induced by varying temperatures may be compensated automatically.

Another object of this invention is .to provide means whereby the air pressure on the face of the crystal or piece-electric oscillator may be reduced to allow free oscillation to a fixed frequency.

Another object is to provide means of preventing the crystal or piano-electric oscillator or resonator from being harmed by the use of high voltages. 7

Other objects and advantages of this invention will become apparent from the following description taken in connection with. the accompanyingdrawing, and will be apparent that many changes may be made in the details of construction and arrangement of parts without departing from). the spi t of the invention as expressed in the accompa y'Jlg clairncf I, thereforado not to be li .lited to the precise details of cons ruction and arrangements shown and described as the preferred forms only have been shown by way of illustration.

' I. is a perspective view of the complete osc War or esonator;

. If is a part section of alternative supthe crystal; 112 is a section on line 3-3 of Fig. I; g

. IV is a section on line 4-4 of Fig. I.

V shows a sectional View of the complete imilar to that of Figure IV with the of means for regulating the air pressure the container. I 111 previous experience it has been found that there are two factors which hinder the correct functioning of a crystal or piezo-electric oscillator or resonator. Gne of these factors is a corona discharge from the plates to the crystal when using high voltages. This discharge causes heating in the crystal and eventually fracture. The other factor is the prevention of free vibration in the crystal by the surrounding air. .The air being of a viscous nature takes up the vibrations given out by the crystal and thereby has a dampening effect. These two factors are the chief drawbacks in piezo-electric crystal oscillators or resonators and the prior art has been concerned with surmounting them. By the use of a partial or sometimes a complete vacuum the prior art has eliminated the fear of heating and fracture and also lowered the frictional effect of the surrounding air upon the piezo-electric crystal oscillator or resonator. In my invention I also make use of an exhaustion of air from the crystal container but in a novel way. 'By a unique arrangement I can control the oscillation of the crystal and regulate this oscillation to any desired frequency, accomplishing this after the crystal is placed in the container.

Prior to my invention the vibration of the crystal oscillator or resonator was uncontrollable when in a partial or complete vacuum. In the prior art if the crystal or piezo-electric oscillator or resonator was placed in a partial vacuum the residue of air in the container affected by varying temperature which reacted on the crystal. by changing its frequency of oscillation. These changes were detrimental to the functioning of the apparatus but in my invention I have compensated for them in such a way as to eliminate any adverse effect.

Referring to the drawing wherein similar reference characters denote corresponding parts 1 throughout, the numeral 1. designates a crystal having piazza-electric properties. The crystal is shown between two metallic plates 15, preferably of brass. On the upper surface of the lower plate are two ridges 16 which having a small surface 9'. area forina preferable form of support for the crystal 1. These supports are plainly shown in Figs. Ill and IV. The upper plate is usually slightly clear of the crystal being kept in position by the insulating strips 2. These insulating strips are fixed by cement or other suitable means between the plates 15 allowing the crystal to vibrate freely. The crystal when. oscillating between the metallic plates is kept in position by pegs 5 which are spaced around it by protruding from the base 6 to a convenient height.

The base 6 is preferably of glass with a recess or clearance for the terminal screw 14. Fixed to the base by cement or other suitable air-tight means is the container 8, preferably of clear transparent material, such as glass or its equivalent. Means for exhausting the air in the container are shown at 9. Wire connections 11 are associated with the plates 15, one wire being drawn through the base and the necessary hole 119 being made air-tightby the use of the usual sealing wire. The other Wire is drawn through the container 8 as shown at 13 and the necessary hole sealed in the same way to make it air-tight. The wire connections 11 may have connection with the plates 15 by means of the screws 14, but other means known to the art may be used.

In the operation of the apparatus the component parts may be assembled as shown in Figs. I, III and IV Previous to this assembly the crystal is placed between the metallic plates and the oscillations in normal .air pressure are adjusted to give a slightly lower frequency than is finally required. This adjustment is accomplish- 15 ed by varying the space between the plates 15 by means of the insulating strips 2. sulating strips also perform another operation of a novel nature. By selecting material of suitable expansion and contraction in varying temperatures, compensation can be made for the change in frequency of the crystal or piezo-electric oscillator in similar varying temperatures.

An alternative construction is shown at Fig. II. The supports 12 are made of a metal having low expansion such as iron. The inner supports 10 are made of a metal having a high expansion such as copper. The crossstrips '7 are preferably of metal and support the inner parts of v the apparatus. The difference in the expansion and contraction of the supports 12 and 10 will compensate for the various types of crystal that are used, some crystals having a greater oscillating degree than others. When placed in the container 8 and in position with the pegs 5 the plates and crystal are ready to receive electrical energy by means of the associated wires 11 from an alternating current. After applying electrical energy the air in the container is exhausted until the required frequency of oscillation in the crystal is gained. The exhaustion of air in the container 8 is carried out by fitting a vacuum pump with tube and stop cocks to the nozzle at 9. The piezoelectric crystal is connected as a resonator or oscillator and the frequency measured during the evacuation of the container. When the correct frequency is found the stop cocks leading to the exhaust apparatus are closed. The sealing process is carried out in the usual way by heating the nozzle 9 between the vacuum pump tube and. the container. The stop cocks and tube may be left on at9 instead of sealing off the-container so that the frequency may be adjusted again at a future time, as shown in Figure V.

If the piezo-electric crystal is used as an oscillator the frequency can be adjusted very easily by beating at an audio frequency with an ordinary oscillating detector. The detector may contain either headphones or audio amplifier and 6 loud speaker, and beats with a standard frequency oscillator. As the piezo-electric crystal approaches the standard oscillator in frequency the audio frequency note, regardless of its frequency, will rise and fall in intensity as the piezo-electric crystal oscillator gains or loses on the standard. Where the audio note takes several seconds to change in intensity the piezoelectric crystal oscillator is within a fraction of 70. a cycle of the standard. There is now a partial These invacuum in the container which will prevent breakage of the crystal oscillator or resonator should high voltage be used and also a corona discharge from plate to crystal which is detrimental to the proper functioning of the appara- 8O tus. As a partial vacuum is subject to changes of temperature the frequency of the crystal will vary but be compensated by a judicious choice of insulating material for the strips between the metallic plates. This forms an extremely simple, automatic and efiicient means of surmounting an obstacle of serious importance to the art. The novelty of having means of controlling the frequency of a crystal or piezo-eleotric oscillator or resonator after placing in a partial vacuum is apparent, and this feature combined with provision for compensating temperature changes makes this invention of wide scope and use.

Having described my invention, I claim:

1. In a device of the character described, a

piezo-electric crystal mounted between two spaced electrodes and a spacing member for holding the electrodes in spaced relation to each other comprising an insulating member supporting spacer bars having physically separated 160 of different material to neutralize the expansion and contraction of temperature changes to hold the constancy of the vibration frequency 0. the crystal one of said arms being comprised wholly of a metal having a high expansion characteristic and the other of said arms being comprised wholly of a metal having a low extension characteristic.

2. The process of maintaining the constancy of vibration'frequency or" a piezo-electric crystal comprising mounting the crystal in an air-tight container, and exhausting the air in the container until a pressure in the container is reached which will permit the crystal to vibrate at the required frequency and regulating the frequency of vibration or the crystal by control of the air pressure in the container.

3. In combination, two spaced electrodes, an element mounted between said electrodes and adapted to vibrate freely, and supporting members for keeping-said'electrodes apart and in spaced relation to each other comprising an inner andv an outer metallic support, said inner support having high expansion and said outer sup port having low expansion characteristics.

. 4. A combination as defined in claim 3 characterized in this, that said inner support is comprised wholly of copper and said outer support wholly of iron. 7

5. A piezo electric crystal apparatus comprising an air-tight container having located therein a crystal element mounted between two spaced electrodes, temperature responsive members separating said two electrodes for compensating for frequency variation due to change in temperature, means for regulating the air pressure with in said container, and a plurality of conductors associated with said crystal at least one of which passes through said container, said one conductor having its entrance point to the container suitably sealed for preventing the atmosphere from affecting the pressure Within said container, or vice versa.

EDGAR D. TILLYER. 

